This invention relates to a method of producing bearings with integral solid-lubrication and more particularly to bearings for use in harsh environments, such as those encountered in an X-ray tube, where organic lubricating agents would be unacceptable.
A principal component of conventional X-ray equipment and computed tomography (CT) equipment is an X-ray tube which provides the source of X-rays. Such tubes contain a vacuum at 10.sup.-8 to 10.sup.-9 torr and operate by accelerating a stream of electrons from a heated cathode through a high voltage against a target anode. The conversion efficiencies of such tubes are low and therefore considerable heat is generated in the anode as a by-product of the x-rays generation.
In order to reduce heat concentration in the anode, the anode is rotated at speeds up to 10,000 RPM thereby continuously presenting the cathode a new and cooler surface. In a high performance x-ray tube, the surface of the anode may reach temperatures of 3200.degree. C., and areas of the anode outside the immediate target surface may rise to temperatures of approximately 1300.degree. C.
Much of the heat generated in the anode is radiated through the glass walls of the tube from high emissivity anode coatings. Even so, the anode support structure, including the support bearings on which it rotates, may rise to temperatures of up to 450.degree. C. It should be noted that the anode support structure, including the bearings, are contained within the evacuated x-ray tube envelope.
The combined effect of high rotational speed, high operating temperatures and a vacuum environment place severe demands on the anode support bearings. Bearing failure is consequently the chief limit to x-ray tube life. Such failure may be either from bearing "weld", a seizure of the rolling elements of the bearing within their race from lack of sufficient lubrication, or from bearing noise which may necessitate tube replacement despite otherwise acceptable operation. Bearing noise is thought to arise from surface irregularities in the bearings and from particulate matter within the race, either foreign substances incorporated into the bearing during the bearing manufacture or particles shed from the the bearings themselves.
The extreme operating temperature of x-ray tubes and their internal evacuated environment precludes the use of conventional organic lubrications on x-ray tube bearings. In this application, organic lubricants would rapidly decompose or evaporate. The industry has therefore directed its attentions to solid lubricants for use with such bearings including metals such as such as lead, silver and gold, and various non-metals such as molybdenum disulfide and niobium disalinide.
The use of metals, in particular silver, as lubricants presents the problem of coating the bearing substrate with lubricant so as to prevent delamination of the lubricant from the bearing substrate under operating conditions. The low adherence that makes silver a good lubricant in tool steel bearings causes it to resist bonding to the bearing elements.
This problem of lubricant adhesion has been addressed in the prior art by the use of intermediary, multiple plated layers of different materials to enhance the bonding between the steel bearing part and the lubricating coating. Progressive electroplated layers of nickel, copper and silver heated to allow mutual diffusion are disclosed in U.S. Pat. No. 4,508,396 entitled: "Method of Producing Bearing Component" issued Apr. 2, 1985. The drawbacks to this approach are that additional steps are required for the multiple electroplating operations and the surface produced by the electroplating is relatively rough and of poor structure. The variations in thickness of each of the multiple layers may also aggregate to distort the profile of the final bearing surface.
Adhesion between the solid lubricant and the bearing substrate may be increased alternatively by the use of abrasives or etchants to roughen the bearing surface thereby increasing the bonding surface area presented to the lubricant. Such roughening, however, works against the ultimate goal of producing a smooth lubricant layer and may necessitate additional plating thickness to improve the ultimate surface finish. As will be described below, it is believed that increased lubrication layer thickness may decrease the lubricating effect of the layer. Further, when the roughening is performed by abrasive blasting, abrasive particles may remain affixed to the bearing, damaging the lubrication layer or becoming unattached thus increasing tube noise and shortening bearing life.